The measurement of color is of great importance in the quality control of many critical intermediates and finished products. Color is not just a physical phenomenon, it is also perception, and perception can not be measured. However, the physical properties involved in the preception of color such as the spectral response of the observer, the reflectance or transmission curves of the sample, and the characteristics of the illuminating source, all can be measured and standardized. Tristimulus colorimetry is an instrumental technique used to assess those characteristics of transmitted or reflected light according to three spectral functions. The theoretical background for this technique was established by the International Commission on Illumination in 1931. The theory, practice and application of tristimulus colorimetry can be surveyed by reference to the following texts: F. Billmeyer and M. Saltzman Principles of Color Technology 1981; R. Johnston and M. Saltzman Industrial Color Technology 1971; and D. Judd and G. Wyszecki Color in Business, Science and Industry 1975.
Tristimulus colorimeters normally employ reflection techniques to measure the color of solid samples. Two optical designs are predominant in this type of instrumentation. One employs an integrating sphere to diffusively illuminate the sample and measure its reflectance curve. Sphere instruments usually include a scanning grating or photodiode array spectral analyzer. A second instrumental design relies on an angular illumination/observation geometry, commonly 45 degree illumination and 0 degree observation. These units use either grating or filter analyzers.
A large perfectly homogeneous, and opaque solid would be close to the ideal type of specimen for color evaluation when contrasted with pelletized or granulated materials. Those nearly ideal solids are easily handled, do not require any sampling device, and yield very precise results. In contrast, pelletized or granulated samples require some type of sample container. Typically, the sample container has a flat optical glass bottom, glass sides and an open mouth at the top. The granular or pelletized sample is poured into the cup and held in place with a plunger inserted into the mouth of the cup. The cup is then placed over the illumination/observation aperture of the tristimulus instrument so that light can be shown through the flat optical glass bottom of the cup, illuminate the sample in the cup and then be reflected back into the instrument for tristimulus analysis. A black cover is usually placed over the sample cup to keep room light from finding its way into the instrument when a sample is being analyzed. If the granular or pelletized sample is translucent and lightly colored, then the use of a conventional sample cup results in poor precision and poor color sensitivity.